Today the surfaces of roads, highways, parking lots and the like are commonly painted with lines or stripes that designate traffic control and parking lanes. In doing so paint, heated thermosetting or reactive setting plastic marking materials, are applied to asphalt and concrete road surfaces. This is done with the use of mobile road marking equipment that has a material hopper from which material is dispensed by means of a spray or extrude apparatus. The screed/extrusion device involves an apparatus commonly referred to in the industry as a die which is mounted on the road marking equipment to be dragged on the raod. The apparatus or die itself has a small hopper in which the preheated marking material is poured from the larger equipmet hopper and then guided through an outlet in the bottom of the die hopper directly onto the surface of the road as the apparatus is advanced over the road surface. These apparatuses or dies commonly have a pair of side runners between which the material contacts the road surface. The runners thus serve both to provide sliding supports for the bottom material flow closable floor die gate as well as to provide side forms which limit lateral flow of the marking material and thereby define the side edges of the marked line. To terminate the end of a line, a valve or sliding floor gate is actuated to close the outlet in the bottom of the die hopper.
In recent years road marking apparatuses or dies have been provided with means for periodically altering the thickness or height of the road marking material. This has been done to provide discontinuities or ridges in the surface plane of the road lines or stripes which in turn enhance the reflectivity of the line from vehicle headlights as a visual aid to motorists. Exemplary of such an apparatus is that shown in U.S. Pat. No. 3,994,611. It employs a vertical slide plate or doctor blade mounted to the rear end of its hopper for reciprocal, vertical movement. In use, marking material that has just flowed out of the die hopper onto a roadway surface between the apparatus runners is beveled by the bottom edge of the trailing doctor blade. As the blade periodically changes its elevation over the road surface the surface elevation of the marking material is changed prior to the material setting and hardening. In this manner steps or ridges are periodically formed along the line at two different elevations above the road surface which steps are connected by sloping sections.
Though road marking apparatuses of the type just described do operate well in varying the height or thickness of road marking materials, such has been done at a sacrifice in making sharp line terminations. This is because apparatuses have used the doctor blade itself to scoop up terminal portions of the marking material from the road surface back into the hopper as shown in the previously mentioned patent. As a result terminal portions of the lines have often had a very thin layer of residual material left on the roadways since their surfaces are rarely smooth but have small pits in which residual material may reside even after having been swept by the doctor blade.
Accordingly, it is seen that a need remains for a road marking apparatus of a type which has the capability of forming road lines of varying heights or thicknesses with improved means for providing clean and sharp line demarcations. It is to the provision of such therefore that the present invention is primarily directed.